There has been a great deal of effort spent in trying to design and build practical, efficient and effective water vapor delivery systems for delivering water vapor at precisely controlled mass flow rates and pressures for use in a variety of applications, including converting harmful chemical byproducts to safer compounds for disposal in an environmentally sound manner. For example, when perfluorocarbons (PFCs), such as CF4 and C2F are used in certain semiconductor fabrication processes, PFC byproducts in the effluent of such fabrication processes must be treated prior to release because they may contribute to the deterioration of the protective ozone layer above the earth's atmosphere. A Plasma reaction of such PFC byproducts with water vapor converts the PFCs to hydrogen fluoride (HF), carbon dioxide (CO2) and water vapor (H2O), which are harmless and can be released, and is thus an attractive method of handling the PFC disposal problem. Water vapor for such reactions may be provided by conventional water vapor deliver systems that function under relatively normal pressure conditions to provide water vapor at or above about 100° C. The water vapor produced by such systems is generally delivered to a plasma reaction chamber by attaching a pump to the vapor delivery system.
There are several drawbacks to using these conventional water vapor delivery systems. First, traditional systems may require about forty watts of power to vaporize one gram of water. Thus, the energy required to vaporize water on a large scale may add significant costs to the manufacturing process. Further, there are a number of problems with metering vapor flow, recondensation of vapor, vapor pressure control, and the like that have contributed to the expense of vapor delivery systems.
A typical water vapor delivery system that may be used for such applications as CFC effluent conversion has an evaporation chamber equipped with a large number of very hot plates with enough surface area to transfer the heat required to vaporize water almost instantaneously to react with and convert the PFC byproducts. Liquid water is fed into the chamber via a liquid metering device at a flow rate suitable to provide just enough water for vaporizing at the desired water vapor delivery rate to the PFC reaction chamber.
There are various drawbacks to using this kind of system. First, the plates have to be maintained at very high temperatures to drive the almost instantaneous evaporation of water flowing into the chamber. This requires significant energy input that may result in increased manufacturing costs.
Second, the high water temperature needed to provide near instantaneous vaporization on very hot surfaces increases the effects of corrosion throughout the system's components, which may result in increased repair and replacement costs. Third, since the liquid flow into the chamber is metered instead of the vapor flow out of the chamber, the actual vapor flow rate out of the chamber may oscillate and prove unstable due to high pressure/temperature fluctuations and evaporation irregularities.
Further, the vapor delivery system requires the maintenance of an elevated temperature throughout all of the components so that vapor pressure will not be exposed to any “cool spots” within the flow route that could cause re-condensation. Further yet, the high temperature system poses a potential safety risk to system operators.
An alternate water vapor delivery system uses a water evaporation chamber to heat a larger quantity of water to a temperature high enough to provide vapor on demand in combination with a vapor or gas mass flow controller (MFC) in a vapor feed line to meter the amount of vapor that is allowed to flow out of the vaporization chamber to the PFC plasma reactor. While this type of system may overcome some of the drawbacks of the previously described system, it is still necessary to keep the entire system (including a relatively large amount of deionized (DI) water) at a continuously high temperature (e.g. between 90° C. and 140° C.), which drives up thermal costs and introduces safety concerns for workers interacting with such systems. Additionally, the duration of time required to keep DI water at an elevated temperature also yields a significant elevation of the corrosive profile of the DI water to a level high enough to adversely affect the system's components.